Boehmitic aluminas, and high-temperature stabile and highly porous aluminum oxides in a pure phase which are obtained therefrom

ABSTRACT

This invention relates to crystalline boehmitic aluminas the crystallites of which exhibit unusual dimensional differences in the space directions 020 and 120. This invention further relates to a method for preparing such aluminas and the follow-up products obtained therefrom by calcination.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to crystalline boehmitic aluminas thecrystallites of which exhibit unusual dimensional differences in thespace directions 020 and 120. This invention further relates to a methodfor preparing such aluminas and the follow-up products obtainedtherefrom by calcination.

2. Description of the Prior Art

The structural relations of the various aluminium oxides and aluminiumhydroxides are very complex. Main distinctions are made between α-Al₂O₃(corundum), α-AlO(OH) (diaspore), α-Al(OH)₃ (occasionally also termedβ-Al(OH)₃, bayerite, or bauxite dihydrate), γ-Al₂O₃, γ-AlO(OH)(boehmite), and γ-Al(OH)₃ (occasionally also termed α-Al(OH)₃, gibbsite,hydrargillite). In addition, there exist numerous modifications thereof,particularly modifications of different aluminium oxides obtained bythermal degradation of the aluminium hydroxides or aluminium oxidehydrates. For instance, it is generally believed that boehmitic aluminawill undergo the following conversion under the influence oftemperature:

Boehmite→γ(gamma)-Al₂O₃→δ(delta)-Al₂O₃→θ(theta)-Al₂O₃→α(alpha)-Al₂O₃

In literature references there are no standardized designations for thevarious aluminium oxides, aluminium oxide hydrates (occasionally alsotermed aluminium oxide hydroxides), and aluminium hydroxides,particularly with respect to the preceding Greek characters. The term‘boehmitic aluminas’ as used herein comprises boehmitic andpseudo-boehmitic aluminas.

Boehmitic aluminas are known. High-purity boehmitic aluminas can beprepared for example by controlled hydrolysis of aluminium alkoxides.The resultant aluminium hydroxide hydrogels crystallize for example inthe form of the rhombic aluminium oxide hydrate crystallite (γ-AlO(OH),boehmitic alumina).

DE 38 23 895-C1 discloses a process for producing boehmitic aluminaswith pore radii which can be adjusted in a controlled way from 3 to 100nm. According to said process, the boehmitic aluminas are subjected tohydrothermal aging at a steam pressure from 1 to 30 bar (correspondingto a temperature from 100 to 235° C.) for 0.5 to hours with agitation ata peripheral velocity from 1 to 6 s⁻¹. Such aluminas and the boehmiticaluminas produced by other processes have crys-tallite sizes (measuredon the 020 reflex) which are always smaller by at least 2 nm compared tothe crystallite sizes measured on the 120 reflex. In U.S. Pat. No.3,898,322, too, a process for producing hydrothermally aged aluminasuspension is described. According to said process, the aqueousaluminium hydroxide/aluminium oxide hydrate suspension obtained byhydrolysis of the aluminium alkoxides is subjected to hydrothermal agingat room temperature for 2 to 60 hours.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide boehmitic aluminashaving unusual morphologies. It is a further object of this invention toprovide aluminium oxides with unusual high-temperature stabilities and,furthermore, with extraordinarily large surfaces and pore volumes aftercalcination.

The problem is solved by crystalline boehmitic alumina with acrystallite size measured in nm on the 020 reflex which is larger thanthe crystallite size which is smaller by 1.5 nm, preferably 0.5 nm,measured on the 120 reflex. It is particularly preferred that thecrystallite size measured in nm on the 020 reflex be larger than thecrystallite size measured in nm on the 120 reflex.

It is a further object of the present invention to provide methods forpreparing the crystalline boehmitic aluminas of the instant invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The starting compounds employed for preparing the crystalline boehmiticaluminas of the present invention are conventional (i.e. crystalline,partially crystalline, or amorphous) aluminium-oxygen compounds, such asaluminium oxide hydrates, aluminium hydroxides, or mixtures thereof withaluminium oxides, preferably conventional pseudoboehmitic and/orboehmitic aluminas. When using commercial aluminium-oxygen compoundsproduced by other processes, or when the boehmitic alumina is notproduced and is readily employed, for example as a hydrogel, it ispreferred that the aluminium-oxygen compounds be subjected to grindingprior to aging according to this invention.

The starting compounds are aluminium oxide hydrates (or aluminium oxidehydroxides) which are preferably prepared by hydrolysis of aluminiumalkoxides obtained from C₁ to C₂₄₊ alcohols or mixtures thereof. Thealuminium alkoxides can be prepared for example by the Ziegler process.

The aluminium alkoxides are hydrolyzed in an aqueous environment.Generally, the hydrolysis can be performed in a temperature range from30 to 150° C., preferably 60 to 100° C. The resultant aluminium oxidehydrate suspension is then separated from the aqueous alcohol phase. Thealumina-water phase may contain for example alumina hydrate with anAl₂O₃ content from 5 to 12 wt. %, preferably 10 to 11 wt. %.

The aluminium-oxygen compounds employed as starting materials may alsooriginate from natural resources or can be produced by other processes,e.g. the amalgam process.

The crystalline boehmitic aluminas of the present invention can beprepared by long-time hydrothermal aging of oxygen compounds of thealuminium, particularly aluminium oxide hydrates, in the presence ofwater at temperatures from 60 to 240° C., preferably 70 to 160° C., mostpreferably 70 to 110° C. for at least 10 hours, preferably at least 20hours, most preferably at least 24 to 70 hours or 30 to 60 hours. It isdesirable to keep the shear stress on the aluminium oxide hydratesuspension low during the production. The term ‘low shear stress’ usedherein means the shear stress caused by an agitator, e.g. a propelleragitator, running at a peripheral velocity of 0.5 to 3 m/s. The particlesize of the aluminium oxide hydrates in the suspension is preferably inthe range from 1 to 12 microns, most preferably from 6 to 12 microns.

According to another embodiment of the present invention, thecrystalline boehmitic aluminas of this invention can be prepared byhydrothermal aging in the presence of water and at least bidentate,preferably at least tridentate bases, which are preferably nitrogenbases, and at temperatures from 30 to 240° C., preferably 70 to 160° C.,for 0.5 to 170 hours. Examples thereof are diethylene triamine,dipropylene triamine, triethylene tetramine (triene), tetraethylenepentamine (tetrene), and pentaethylene hexamine (pentrene).

According to yet another embodiment of the present invention, thecrystalline boehmitic aluminas of this invention can be prepared bylong-time hydrothermal aging in the presence of water and metallic ornonmetallic oxides, or oxide hydrates, except for aluminium oxide oraluminium oxide hydrates, and water at 40 to 240° C., preferably 70 to160° C., for at least 8 hours, preferably 16 to 170 hours, mostpreferably 32 to 170 hours.

Preferably, said metallic or nonmetallic oxides or oxide hydrates arethose of silicon, zirconium, titanium, lanthane, and/or boron. Examplesthereof are SiO₂, ZrO₂, TiO₂, and B₂O₃. Such oxides are added inquantities from 0.1 to 5 wt. %, preferably 0.2 to 2 wt. %, referring toA1₂O₃.

It is preferred that the crystalline boehmitic aluminas of the presentinvention or the aluminium oxides prepared therefrom be free from anyforeign atoms, particularly other metal atoms (including silicon andphosphorus), i.e. said materials should exclusively consist ofaluminium, oxygen, and/or hydrogen in quantities of greater than 99 atom%, preferably greater than 99.9 atom %.

Preferably, the crystalline boehmitic aluminas of the present invention,independently of one another, have the following characteristics: porevolumes of greater than 0.8 cm³/g, preferably greater than 0.9 cm³/g,crystallite sizes (measured on the 020 reflex) of greater than 10 nm,preferably greater than 12 nm, and surfaces of greater than 150 m²,preferably 150 to 200 m². In contrast thereto, it is preferred that thecrystalline boehmitic aluminas of the present invention preparedaccording to the third embodiment, independently of one another, havethe following characteristics: pore volumes of greater than 0.7 cm³/g,preferably greater than 0.9 cm³/g, crystallite sizes (measured on the020 reflex) from about 6 to 10 nm, and surfaces of greater than 200 m².

The aluminium oxides prepared from the crystalline aluminas of thepresent invention by thermal treatment at higher than 150° C.,preferably by calcination at temperatures from 800 to 1,500° C. for atleast 0.5 hour are a further object of the present invention. Saidaluminium oxides are distinguished by their particularly large surfaces,large pore volumes, and excellent high-temperature stabilities. The term‘thermal stability’ employed herein means stability to changes in thesurface or crystalline phase brought about by external influences, suchas water, chemicals, pressure, or mechanical stress and temperature.

Furthermore, the aluminium oxide hydrates and aluminium oxides accordingto the present invention are pure-phase and stable-phase products whichare present as delta, theta, or alpha modifications, depending on thecalcination time and temperature. More details are presented in thetables 1, 2, and 3 showing the powder diffraction pattern data of thedifferent aluminium oxides of this invention.

The term ‘pure-phase’ employed herein means that more than 90 wt. %,preferably more than 98 wt. % of the crystalline aluminium oxideconsists of a single phase (determined by X-ray powder diffraction). Thetheta-aluminium oxides of the present invention are pure-phase productson the condition that particularly the d-values (as Å) do not presentany peaks in the X-ray powder diffraction pattern which arecharacteristic of α-Al₂O₃.

With respect to the X-ray powder diffraction patterns of conventionalaluminium oxides, reference is made to the corresponding. JCPDS sheets(US National Bureau of Standards) for corundum (α-Al₂O₃), delta- andtheta-aluminium oxide.

The term ‘stable-phase’ employed herein means that the crystalline phasewill not change even if the product is exposed for a long time to thesame or lower temperature used in the production of said aluminium oxideby calcination.

Furthermore, the aluminium oxides of the present invention aretemperature-stable and, contrary to conventional aluminium oxides, havesurfaces of larger than 60 m²/, preferably larger than 70 m²/g, evenafter calcination at 1,200° C. for 3 hours. The calcination is performedin heated air in a muffle furnace.

The aluminium oxides of the present invention have pore volumes ofgreater than 0.6 cm³/g, preferably from 0.7 to 1 cm³/g (determined bythe mercury penetration method in accordance with DIN 66 133) within apore radius range from 1.8 to 100 nm. The aluminium oxides of thisinvention keep said characteristic even after exposure to temperaturesof 1,100° C. for 24 hours. Conventional aluminium oxides, e.g. thoseobtained by calcination of bayerite, present distinctly smaller porevolumes (about 0.2 to 0.4 cm³/g).

The aluminium,oxides of this invention are most useful as catalysts orcatalyst supports, particularly as support material for automobileexhaust gas catalysts. In this case the catalyst support is treated withnoble metal catalysts, such as platinum or palladium.

When using the aluminium oxides of this invention, the catalyst orcatalyst support can be applied in thin layers which remain stable evenat high temperatures, e.g. of greater than 1,000° C. This characteristicis most advantageous in exhaust gas catalyst applications. Furthermore,stabilization aids, such as lanthane oxideor SiO₂, employed in technicalapplications can mostly be dispensed with. Stabilization aids made ofmetal oxides may have adverse effects on the catalytic behavior of theAl₂O₃ catalyst or catalyst support.

The crystallite sizes of the boehmitic aluminas according to thisinvention were determined on the 120 and 020 reflexes using the generalScherrer formula:

 Crystallite size=(K×lambda×57.3)/(beta×cos theta)

K (form factor): 0.992

Lambda (X-ray wave length): 0.154 nm

Beta (corrected line broadening of apparatus): reflex-dependent

Theta: reflex-dependent

The measurements were carried out using a Philips XRD X'pert apparatus.The measurement parameters for the samples obtained in Example 1(Comparative Example) and Example 2 have been compiled in Tables 1 and2, respectively.

The reflexes 120 and 020 (Miller indices) were determined on theboehmite and relate to the unconventional crystallographic Amam mountingof orthorhombic space group no. 63. The conventional mounting is Cmcn,wherein the a- and c-axes have been exchanged in comparison with theunconventional Amam mounting.

The surface areas of the aluminium oxides of this invention weredetermined by the N₂ sorption method (BET method in accordance with DIN66131). The pore volumes and pore volume distributions were determinedby the mercury intrusion (penetration) method in accordance with DIN66133 using a mercury porosimeter. The pore volumes were reported ascumulative volumes in cm³/g in accordance with DIN 66133.

EXAMPLE 1 Comparative Example

First, an alumina slurry to be employed as the starting material wasprepared by neutral aluminium alcoholate hydrolysis:

An aluminium alcoholate mixture obtained as an intermediate in theZiegler/Alfol process was hydrolyzed with water which had been liberatedfrom foreign ions in a demineralization unit. The hydrolysis wasperformed at 90° C. in an agitated kettle. The resultant two phases,i.e. the upper alcohol phase and the lower alumina/water phase, wereimmiscible.

500 grams of this alumina slurry (pH 9) containing 10 to 11 wt. % Al₂O₃were added in portions to a reactor operated at a pressure of 3 barcorresponding to 115° C. After the reaction conditions had beenadjusted, the slurry was allowed to age for 4 hours using a standardagitator running at a peripheral velocity of 1.6 m/s corresponding to anagitator speed of 500 r.p.m.

The following values were obtained:

Reflex Beta Theta Crystallite Size 120 0.919 14° 9.8 nm 020 0.919  7°6.7 nm

The 120 reflex is greater by 3.1 nm than the 020 reflex.

The specific surface was determined by the N₂ sorption method (BETmethod). After thermal treatment at 1,200° C. for 3 hours the specificsurface was found to be 46 m²/g. The X-ray powder diffraction pattern ofsaid sample is shown in Table 1. It presents significant alpha-phasesignals.

EXAMPLE 2

500 grams of the alumina slurry (pH 9) containing 10 to 11 wt. % Al₂O₃as defined in the Comparative Example were added in portions to areactor operated at normal pressure and 98° C. After the reactionconditions had been adjusted, the slurry was allowed to age for 16 hoursusing a standard agitator running at a peripheral velocity of 1.6 m/scorresponding to an agitator speed of 500 r.p.m. The crystallite sizesmeasured as described in the Comparative Example were 13.5 nm (120reflex) and 12.1 nm (020 reflex).

After thermal treatment at 1,200° C. for 3 hours the specific surfacewas found to be 68 m²/g. The X-ray powder diffraction pattern of thissample is shown in Table 1. It presents theta-phase signals. Thealuminium oxide is present in the theta phase with a phase purity ofgreater than 98%.

After aging for 20 hours under the conditions specified hereinabove thecrystallite sizes were found to be 13.5 nm (120 reflex) and 13.0 nm (020reflex).

EXAMPLE 3

500 grams of the alumina slurry (pH 9) containing 10 to 11 wt. % Al₂O₃as defined in the Comparative Example were added in portions to areactor operated at a pressure of 3 bar corresponding to 110° C. Afterthe reaction conditions had been adjusted, the slurry was allowed to agefor 40 hours using a standard agitator running at a peripheral velocityof 1.6 m/s corresponding to an agitator speed of 500 r.p.m.

The crystallite sizes measured as described in the Comparative Examplewere 15.3 nm (120 reflex) and 15.3 nm (020 reflex). After thermaltreatment at 1,200° C. for 3 hours the specific surface was found to be67 m₂/g.

EXAMPLE 4

500 grams of the alumina slurry (pH 9) containing 10 to 11 wt. % Al₂O₃as defined in the Comparative Example were added in portions to areactor operated at a pressure of 3 bar corresponding to 110° C. Afterthe reaction conditions had been adjusted, the slurry was allowed to agefor 60 hours using a standard agitator running at a peripheral velocityof 1.6 m/s corresponding to an agitator speed of 500 r.p.m.

The crystallite sizes measured as described in the Comparative Examplewere 16.1 nm (120 reflex) and 16.5 nm (020 reflex). After thermaltreatment at 1,200° C. for 3 hours the specific surface was found to be72 m²/g.

EXAMPLE 5

600 grams of the alumina slurry (pH 9) containing 10 to 11 wt. % Al₂O₃as defined in the Comparative Example were added to 50 grams of a 20%aqueous tetrene solution and boiled under reflux for 68 hours. 300 gramsof H₂O were added to this mixture at one-hour intervals. The mixture wasdiluted with 200 grams of H₂O und spray dried.

The crystallite sizes measured as described in the Comparative Examplewere 14.4 nm (120 reflex) and 14.6 nm (020 reflex). After thermaltreatment at 1,200° C. for 3 hours the specific surface was found to be81 m²/g.

EXAMPLE 6

300 grams of a 6.02% aluminium-tri-n-hexanolate solution in n-hexanolwere added at 90° C. to 360 grams of a 5% aqueous tetrene solution. Thismixture was agitated at 90° C. for 30 minutes. The hexanol was removedfrom the reaction mixture by azeotropic distillation. The residue thenwas agitated at 90° C. for 24 hours. H₂O was added in 100-gram portionsafter 1 hour, 2 hours, and 3 hours, respectively. The reaction mixturewas spray dried.

The crystallite sizes measured as described in the Comparative Examplewere 11.0 nm (120 reflex) and 11.8 nm (020 reflex). After thermaltreatment at 1,200° C. for 3 hours the specific surface was found to be76 m²/g.

EXAMPLE 7

300 grams of a 6.02% aluminium-tri-n-hexanolate solution in n-hexanolwere added at 90° C. to 360 grams of a 5% aqueous tetrene solution. Thismixture was agitated at 90° C. for 30 minutes. The hexanol was removedfrom the reaction mixture by azeotropic distillation. The residue thenwas agitated at 90° C. for 68 hours. H₂O was added in 100-gram portionsafter 1 hour, 2 hours, and 3 hours, respectively. The reaction mixturewas spray dried.

The crystallite sizes measured as described in the Comparative Examplewere 12.6 nm (120 reflex) and 16.4 nm (020 reflex). After thermaltreatment at 1,200° C. for 3 hours the specific surface was found to be79 m²/g.

TABLE 1 X-Ray Powder Diffraction Pattern of Example 1 (ComparativeExample) with Significant Quantity of Alpha-Al₂O₃ d-Value d-Valued-Value T-Width Height Backgr. Rel. Int. [°2θ] α¹ [Å] α² [Å] [°2θ][counts] [counts] [%] Signific. 16.205 5.46527 5.47872 0.480 19 29 4.11.56 19.550 4.53706 4.54822 0.480 45 27 9.5 2.53 21.850 4.06440 4.074390.480 9 24 1.9 0.80 25.560 3.48224 3.49081 0.180 276 30 58.5 7.44 31.2302.86174 2.86878 0.200 286 41 60.7 2.47 32.695 2.73678 2.74351 0.120 44137 93.7 1.23 32.825 2.72624 2.73294 0.080 441 37 93.7 0.97 35.1352.55211 2.55838 0.140 449 35 95.4 4.34 36.660 2.44937 2.45539 0.200 31334 66.5 1.41 37.695 2.38446 2.39032 0.140 185 31 39.3 2.48 38.9102.31275 2.31844 0.120 266 30 56.4 0.91 39.855 2.26007 2.26563 0.160 16930 35.9 0.92 41.715 2.16349 2.16881 0.480 21 27 4.5 1.69 43.305 2.087672.09280 0.160 396 27 84.1 6.77 44.785 2.02205 2.02703 0.320 282 26 59.99.47 45.605 1.98758 1.99247 0.360 137 25 29.1 3.48 46.515 1.950801.95560 0.240 90 24 19.2 2.18 47.630 1.90770 1.91239 0.120 185 23 39.31.31 50.680 1.79981 1.80424 0.320 77 22 16.4 4.82 51.520 1.77242 1.776780.320 22 22 4.7 0.78 52.450 1.74317 1.74745 0.100 154 22 32.7 1.1857.415 1.60366 1.60761 0.160 317 25 67.3 6.46 58.730 1.57085 1.574710.240 26 25 5.5 0.76 59.820 1.54480 1.54861 0.120 114 25 24.3 1.9361.240 1.51234 1.51606 0.160 74 25 15.7 1.05 62.295 1.48924 1.492910.400 71 24 15.0 2.02 63.850 1.45667 1.46025 0.240 137 24 29.1 3.2864.165 1.45028 1.45385 0.120 98 24 20.8 1.06 65.450 1.42488 1.428380.320 59 23 12.6 1.06 66.475 1.40537 1.40883 0.100 324 24 68.8 0.8567.395 1.38841 1.39182 0.440 471 23 100.0 18.43 68.155 1.37477 1.378150.100 190 23 40.4 0.92 72.900 1.29654 1.29973 0.320 41 21 8.7 1.3373.700 1.28443 1.28759 0.400 48 21 10.1 2.55 75.460 1.25878 1.261880.560 28 19 6.0 4.30 76.800 1.24012 1.24317 0.160 58 18 12.3 1.06Measurement parameters: Start angle [°2θ]: 5,010; end angle [°2θ]:79.990; start d-value [Å]: 17.62435; end d-value [Å]: 1.19850; anodematerial: Cu; α¹ wave length [Å]: 1.54060; α² wave length [Å]: 1.54439

TABLE 2 X-Ray Powder Diffraction Pattern of Example 2 (Al₂O₃ with > 98%Theta Phase) d-Value d-Value d-Value T-Width Height Backgr. Rel. Int.[°2θ] α¹ [Å] α² [Å] [°2θ] [counts] [counts] [%] Signific. 16.240 5.453575.46699 0.400 40 28 6.6 1.55 19.495 4.54974 4.56093 0.400 56 30 9.4 1.2125.230 3.52704 3.53571 0.800 11 28 1.8 1.26 31.130 2.87070 2.87777 0.320292 42 48.7 4.94 32.685 2.73759 2.74433 0.180 600 42 100.0 3.57 32.7902.72907 2.73578 0.060 562 42 93.6 0.80 34.845 2.57268 2.57901 0.320 10241 17.0 2.05 36.635 2.45098 2.45701 0.320 372 40 62.1 5.21 38.8202.31791 2.32361 0.280 266 41 44.3 4.26 39.890 2.25816 2.26372 0.240 19640 32.7 1.80 43.295 2.08813 2.09326 0.320 15 29 2.5 1.01 44.720 2.024842.02982 0.360 361 28 60.1 12.86 46.530 1.95020 1.95500 0.480 59 27 9.90.93 47.585 1.90940 1.91410 0.280 196 25 32.7 5.21 50.585 1.802971.80741 0.240 85 24 14.1 2.69 51.470 1.77403 1.77839 0.320 31 23 5.21.18 52.455 1.74301 1.74730 0.320 22 23 3.7 1.03 56.580 1.62533 1.629330.640 18 23 2.9 0.99 57.365 1.60494 1.60889 0.320 20 24 3.4 0.75 58.7551.57024 1.57410 0.240 37 24 6.2 2.28 59.850 1.54410 1.54790 0.200 139 2423.2 1.67 62.310 1.48892 1.49258 0.400 100 24 16.7 2.83 63.925 1.455141.45872 0.400 164 25 27.3 10.08 65.355 1.42672 1.43023 0.480 69 26 11.52.58 66.450 1.40584 1.40930 0.240 228 25 38.0 2.47 67.395 1.388411.39182 0.600 524 26 87.4 39.85 72.845 1.29738 1.30057 0.480 50 24 8.42.92 73.715 1.28420 1.28736 0.400 64 23 10.7 3.36 75.295 1.26113 1.264230.400 27 20 4.5 1.56 77.320 1.23308 1.23611 0.960 15 18 2.5 3.48Measurement parameters as specified in Table 1.

TABLE 3 X-Ray Powder Diffraction Pattern of an Aluminium Oxide of thisInvention in the Delta Phase d-Value d-Value d-Value T-Width HeightBackgr. Rel. Int. [°2θ] α¹ [Å] α² [Å] [°2θ] [counts] [counts] [%]Signific. 19.605 4.52446 4.53559 0.480 21 49 6.2 0.82 32.855 2.723822.73052 0.560 69 139 20.3 2.95 36.865 2.43621 2.44221 0.320 114 132 33.80.95 39.570 2.27568 2.28128 0.880 117 90 34.5 10.25 45.475 1.992961.99787 0.560 272 58 80.4 7.14 46.385 1.95596 1.96077 0.480 146 56 43.22.24 50.865 1.79370 1.79811 0.800 10 40 3.0 1.35 58.990 1.56455 1.568390.140 11 61 3.2 0.85 59.985 1.54095 1.54474 0.800 18 14 5.5 1.38 66.4701.40547 1.40892 0.560 279 83 82.4 3.34 67.220 1.39160 1.39502 0.240 33976 100.0 0.99 Measurement parameters as specified in Table 1.

What is claimed is:
 1. A method of preparing crystalline boehmiticalumina characterized in that aluminum-oxygen compounds arehydrothermally aged in the presence of water and at least one bidentatebase at 30 to 240° C. for 0.5 to 170 hours.
 2. A method of preparingcrystalline boehmitic alumina characterized in that aluminum-oxygencompounds are, hydrothermally aged in the presence of water and oxidesor oxide hydrates of zirconium, titanium, lanthanum and/or boron at 40to 240° C. for at least 8 hours.
 3. The method of any of claims 1 or 2characterized in that the boehmitic alumina is subjected to thermaltreatment by calcination to obtain an aluminum oxide having a surfacearea of greater than 60 m²/g.
 4. The method of any of claims 1 or 2wherein the boehmitic alumina is subjected to thermal treatment bycalcination and the resulting aluminum oxide has pore volumes of greaterthan 0.6 cm³/g determined by the mercury penetration method in the poreradium range of from 1.8 to 100 nm.
 5. The method of any of claims 1 or2 wherein the boehmitic alumina is subjected to thermal treatmentcomprising calcination at a temperature of from 800 to 1500° C. for atleast 0.5 hour.
 6. The method of claim 1 further comprising the step ofthermal treatment of the crystalline boehmitic alumina to produce a puretheta aluminum oxide.
 7. The method of claim 2 further comprising thestep of thermal treatment of the boehmitic alumina to produce pure thetaaluminum oxide.
 8. The method of any of claims 1 or 2, wherein thealuminum-oxygen compounds used as starting materials are obtained byhydrolysis of at least one aluminum alkoxide compound.
 9. The method ofclaim 1 or claim 2, wherein the crystalline boehmitic alumina has acrystallite size that determined on the 020 reflex in nm is larger thanthe measured crystallite size determined on the 120 reflex reduced by1.5 nm.
 10. The method of claim 1 or claim 2, wherein the crystallineboehmitic alumina has a crystallite size that determined on the 020reflex in nm is larger than the measured crystallite size determined onthe 120 reflex reduced by 0.5 nm.